In recent years, the practicability of a so-called functional water made by dissolving specific gas in ultrapure water, in a use of washing electronic material has been recognized, and it becomes popular, for example, in a wet cleaning process. An apparatus having a gas permeable membrane module to dissolving gas in the ultrapure water is usually used as a gas dissolving device.
It is known that about the gas dissolving device, regarding gas such as hydrogen, functional water in which gas is dissolved with an intended concentration can be accurately obtained by supplying ultrapure water at a fixed flow rate with respect to a unit flow rate of gas to be dissolved. Therefore, a gas dissolving device is proposed which is configured such that the supply flow rate of the ultrapure water and the supply flow rate of the gas to the dissolving device are controlled to stably keep the concentration of the gas in the functional water.
Here, the ultrapure water in which the gas is dissolved is generally produced in an ultrapure water producing apparatus having a primary pure water system and a secondary pure water system. The primary pure water system produces primary pure water by removing impurities such as ionic substances and nonionic substances from raw water. The primary pure water is stored once in a primary pure water tank, and is introduced from the primary pure water tank into the secondary pure water system. The secondary pure water system treats the primary pure water to produce ultrapure water. As needed, an ultrapure water tank that stores the ultrapure water is placed at the final stage of the ultrapure water producing apparatus. The ultrapure water is stored, for example, in the ultrapure water tank, and then supplied to a point of use of the ultrapure water (POU). The ultrapure water tank and the primary pure water tank are connected by a reflux pipe. The reflux pipe refluxes the ultrapure water from the ultrapure water tank to the primary pure water tank. The point of use of the ultrapure water is connected, for example, to the route of the reflux pipe.
The gas dissolving device is generally connected to the point of use of the ultrapure water. The gas dissolving device dissolves gas in the ultrapure water supplied from the ultrapure water producing apparatus to produce functional water. The produced functional water is directly supplied to the point of use of the functional water. The point of use of the functional water is a washing apparatus for a silicon wafer or the like.
Here, the reflux pipe connected to the ultrapure water producing apparatus sometimes connects to a plurality of points of use. In such a case, the water pressure of the ultrapure water produced by the ultrapure water producing apparatus is usually kept almost constant, and therefore the supply pressure of the ultrapure water supplied to each of the points of use of the ultrapure water, is also kept almost constant.
However, the amounts of the ultrapure water used at the plurality of points of use of the ultrapure water may independently fluctuate. In this case, depending on the timing of fluctuation, the supply pressure of the ultrapure water at a certain point of use of the ultrapure water fluctuates. For example, in the case where the plurality of points of use of the ultrapure water are a plurality of the above-described washing apparatuses, when the timings of stop of washing cycles of some of the plurality of washing apparatuses are overlapped, the supply pressure of the ultrapure water in a not-stopped apparatus sometimes greatly increases. Besides, when the number of points of use of the ultrapure water is large, the point of use of the ultrapure water placed closer to the downstream of the reflux pipe is likely to be affected by the change in the amount of the ultrapure water used on the upstream side, and there are concerns about unexpected decreases in supply flow rate and supply pressure of the ultrapure water.
Incidentally, a silicon wafer increasingly becomes large in size. The large-sized silicon wafer is washed by a pouring washing method of washing the silicon wafer by pouring the functional water thereon. In the washing apparatus which wash the silicon wafer by the pouring washing method, generally, the washing is controlled by the washing time, and therefore when the supply pressure of the functional water to the washing apparatus decreases, the supply amount of the functional water to the silicon wafer surface becomes insufficient, and washing cannot be sufficient in some cases.
The problem of insufficient washing due to the insufficiency in the supply amount of the functional water onto the silicon wafer surface becomes more conspicuous in a larger semiconductor wafer. Specifically, the above problem of insufficient washing becomes more conspicuous, for example, when the silicon wafer is as large as 200 mm diameter or more. Therefore, it has been required in the washing of such a large-sized silicon wafer that the supply pressure of the functional water is more stably kept constant.